The long range goal of these investigators is to determine the influence of the actin-bound divalent cation and bound nucleotide: (1) on the characteristics of both monomer and polymer actin; (2) on the polymerization mechanism of actin; and (3) on the interaction of actin with actin-associated proteins. The research will specifically address the effects of actin-associated proteins on the characteristics and polymerization properties of actin that they have been studying. The specific aims are as follows: (1) the capping of actin polymers by gelsolin and gCap39 will be studied; (2) the mechanism of actin filament severing by gelsolin will be investigated under physiological conditions; (3) the effect of gelsolin on the nucleation of actin polymerization will be clarified; (4) the effect of profilin on actin-bound nucleotide exchange and on actin polymerization will be studied; (5) the influence of thymosin beta4 on bound nucleotide exchange, on actin, and on actin polymerization will be determined; (6) actin filament dynamics and actin-bound nucleotide exchange in ensembles of actin and combinations of all three actin-associated proteins, gelsolin, profilin and thymosin beta4 will be investigated.